Secondary electron emitter and method of making it



June 11, Q KRENZiEN 2,204,252

SECONDARY ELECTRON EMITTER AND METHOD OF MAKING IT Filed July 8, 1939 Fgqi ' A ELECTRODECORE 0F METAL WETCORE WITH AQUEOUS B 0R ALCOHOLIC SOLUTLONOF ALKAL/ METAL HAL/DE TO FORM THIN LAYER 0F HAL/DE 0N CORE HEAT METAL CORE v VACUUM lN/NERT GAS FUSE HAL/DE sN T0 ADHEREN T FUSED HAL/DE LA YERA PROPER FRACTION OFA MICRON THICK EXPO-5E FUSED HAL/DE LAYER T0 VAPOR 0F ALKALI METAL 2 LAYER 0F FUSED ALKALI ETAL HAL/DE A PROPER FRACTION OFA MICRON THICK INVENTOR. 07'7'0 KRENZIEN ATTORNEY.

Patented June 11, 1940 sEcoNnAaY ELECTRON EMITTER AND rmrnon or MAKING 1r 1 Otto Krenzien, Berlin-Siemensstadt, Germany,

assignor to Fidea Gesellschaft fur die Verwaltung und Verwertung von gewerblichen Schutlrechten m. b. 11., Berlin, Germany Application July 8,

1939, Serial No. 283,469

Germany July 28, 1938 drawing in which Figure 1 indicates steps of the method of making the emitter, and- Figure -2 15 shows in greatly enlarged cross-section a .por-.'

tion of a secondary electron emitting electrode'.

In accordance with the invention the segqpgary; electron emitting electrode comprises a body or sheet of metal, such ms tungsten, molybdenum,

20 nickel, or the nickel-copper alloy known as, constantan, with a coating or effective covering consisting of an alkali metal fluoride, for example, sodium fluoride, potassium fluoride, or lithium fluoride in the form of a film or very thin layer 25 about one tenth of a micron or one te'n'thoustandths of a millimeter thick. Hitherto it was the accepted view that layers of alkali metal fluoride would not be useful in practice for high secondary electron emission because at 'very small load currents the layers were destroyed, or'fat least their property of secondary electron emission was detrimentally affected. Apparently such detrimental eifects were due to the fact that the layers used were not sufliciently thin.

36 Such effects are not found with the secondary electron emitting layers made according to the invention, as these very thin layers, less than a micron thick, have been found to be capable of producing a high and practically constant sec- 40 ondary electron emission during long periods of operation even if the current density of the secondary electron current is comparatively high. It was found that these very thin layers have a high secondary electron emissivity, the ratio of 45 secondary electrons emitted to primary electrons received being several times unity upon impact by primary electrons at several hundred volts, and that this emissivity may often be insults are obtained by causing caesium vapor to act upon a fused layer of lithium fluoride, par- .5 tlcularly where care is taken that the amount of alkali metaL'such as caesium, absorbed by the layer is limited and is kept below the saturation point. Apparently the atoms of the vaporized metal reduce in the reaction equilibrium a part of the molecules of the layer, and probably the grid 6 effect thus caused contributes to obtaining the high yield of secondary electrons.

The manner in which the layers are applied to the metal of the electrode is of some importance, asr-it has been found advantageous to heat the 10 coated electrode until the alkali -metal halide melts andfonns on the surface of the electrode a very thin firmly adherent dense and uniform layer of fused alkali metal fluoride which resembles a very thin coat of fused enamel. The secondary emission layers may to advantage be made by moistening the metal electrode with an aqueous or an alcoholic solution ,of the alkali metal fluprides. Although the alkali metal fluoridea'are in general not very soluble in alcohol, a saturated alcoholic solution of the fluorides may be used to advantage, as the layer thus produced is free from water of crystallization. The moistened electrode is dried to leave on it a uniform deposit of fluoride, and then heated to '25 'melt the fluoride deposit into a fused layer or coating on the metal.

Secondary electron emitters having a very thin active layer made according to the invention have on test withstood secondary electron currents 80 or loads up to 20 milliamperes per square centimeter for thousands of hours of operation without losing to any appreciable degree their properties of secondary electron emission, and have shown a secondary electron emission ratio of between two to four secondary electrons per pri-' mary electron with an average secondary elec--. tron current of about 60 milliamperes. In the operation of tubes having electrodes with layers made according to the invention the conditions of operation should be so chosen and the load should be such that during operation the temperature of the secondary electron emitter does not come too close to the melting point of the alkali metal fluoride of the electrode. The metal base for 46 layers consisting of alkali metal fluorides should not during operation attain a temperature above 600 C. If these conditions of operation are observed, a favorable secondary electron emissivity which is high and remains substantially 50 constant during long periods of operation and for thousands of hours can be expected.

A further advantage is obtained if the heating -of' the secondary electron emitting electrode and the melting of the fluoride is carried out in -a protective atmosphere of inert gas such as argon.

' ticles of the layer will not sublime or be thrown off and deposit on the other electrodes, suchas the oxide coated cathode, thereby causing a deactivation of the cathode. Moreover, heating in a protective inert gaseous atmosphere often produces an activation which is somewhat greater and a higher secondary electron emissivity is obtained by heating in vacuum.

Secondary electron emitters made according to the invention may be used in the various ways known in the art for utilizing such emitters, as for example, in electron multipliers, such as shown in U. S. patents to Slepian, No. 1,450,265 or Piore, No. 2,123,024, or in tubes such aspshown in U. S. patent to Hull, 1,721,395 in which the secondary electron emitters are radial plates parallel to the axis of the tube and each stage comprises elongated radially positioned secondary electron emitters.

I claim:

1. A secondary electron emitting electrode having a ratio of secondary electron emission several times unity upon impact by primary electrons at several hundred volts which comprises a metal core and asecondary electron emitting surface layer a fraction of a micron thick adherent to said core and consisting of an alkali metal fluoride.

2. A secondary electron emitting electrode having a ratio of secondary electron emission several times unity upon impact by primary electrons at several hundred volts which comprises a metal core and a layer of fused alkali metal fluoride layer a fraction of a micron thick on and firmly adherent to said core.

3. The method of making a surface having high seam secondary electron emissivity which consists in moistening the surface of a metal body with a volatile vehicle which carries an alkali metal flu oride, removing the volatile vehicle and leaving a deposit of said fluoride on said surface, and melting said deposit into a very thin uniform coating of fusedfluoride firmly adherent to said metal.

4. The method of making a surface having high secondary electron emissivity which consists in forming on the surface of a metal body a substantially uniform deposit of an alkali metal fluoride, and heating said deposit of fluoride intensely for a brief period until said fluoride melts and forms over said surface a firmly adherent uniform coating a fraction of a micron 'thick.

5. The method of making a surface having high secondary electron emissivity which consists in forming on the surface of a metal body a substantially uniform deposit of analkali metal fluoride, establishing an inert protective atmosphere about said body, and melting sa-id deposit in said inert atmosphere to form a very thin uni-.-

form layer of fused fluoride on said surface.

6. The method of making a surface having high secondary electron emissivity which consists in producing on the surface of a metal body a uniform alkali metal fluoride layer a fraction of a micron thick and introducing alkali metal into said layer by exposing saidflayer to the vapor of an alkali metal.

30 7. A secondary electron emitting electrode havseveral ing a ratio of secondary electron emission times unity upon impact by primary electrons at several hundred voltswhich comprises a metal core and a secondary electron emitting surface a layer of lithium fiuoridesubstantially one-tenth micron thick on and adherent to said core.

O'I'IO K'BENZIEN. 

